Need PR Unit should not equal to Material Base UOM

Hi All,

I want to maintain Base UOM as 'PC' in material master. Then i want 'BAG' unit should appear while creating PR & PO.

here 1 BAG = 100 PC .

After i maintained 'PC' uom in material master, the uom 'PC' is appearing automatically while creating PR . This field is not editable in PR.

Please advice what i should do so that i can maintain PC as Base unit in material master & i can use unit 'BAG' for PR & PO.

Thanks

pabi